Gain control of limiter stage of a quadrature grid detector



Dec. 24, 1963 L. J. MATTINGLY 3,115,606

GAIN CONTROL OF LIMITER STAGE OF A QUADRATURE GRID DETECTOR Filed Nov.6; 1956 Amp,

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United States Patent GAIN CGNTROL 0F LIMITER STAGE OF A QUADRATURE GRIDDETECTOR Lawrence J. Mattingly, Lombard, Iil., assignor to Motorola,Inc, Chicago, Ill., a corporation of Illinois Filed Nov. 6, was, Ser.No. 628,674

10 Clm'ms. (Cl. 325-487) This invention relates to demodulator circuitsand more particularly to a detector for frequency modulation signalssuch as the sound carrier of a composite television signal. Reference tofrequency modulation in the speci fication and claims is intended torefer broadly to all angular modulation including phase modulation.

One form of frequency modulation detector employs a single electronvalve with a control element driven by the signal to be detected and afurther control element connected to a resonant circuit tuned to thesignal. This resonant circuit is energized by the signal applied throughcoupling within the tube and a wave developed therein regulatesconduction of the tube in the region near quadrature or zero passage ofthe signal. As the modulated signal deviates, the phase angle at whichthe further control element is driven changes with respect to the drivesignal and the modulation of the signal is available at the outputelement of the valve.

A circuit of this type can be used as a simple detector for the soundsignal in a television receiver. However, in such service there may be agreat variation in signal level applied to the detector and this cancause undesirable operation. For example, in fine tuning a televisionreceiver so that the video carrier of the composite television signal isnear the maximum response of the intermediate frequency response curve,the sound carrier could be of the order of 60 db below the carrierlevel. In this situation the comparatively weak sound carrier may not beof sufficient strength to properly operate the detector, that is, it maynot hold the detector locked in to follow deviations thereof thusproducing distortion. On the other hand, if the sound carrier ispositioned near the maximum response of the intermediate frequencycurve, this signal can be greater than the video carrier by an amount ofthe same order. This condition can cause overdrive of the detector andadversely affect the amplitude modulation rejection of the detector.

It is accordingly an object of this invention to provide a detectorsystem for frequency modulation signals which operates in an improvedmanner over a wide range of signal levels.

Another object is to provide a detector system for frequency modulationsignals which has increased sensitivity for weak signals and decreasedsensitivity for strong signals.

A further object is to provide a frequency modulation detector whereindistortion of weak signals is reduced and amplitude modulation rejectionat high signal levels is greatly enhanced.

A feature of the invention is the provision of a simple frequencymodulation detector system which automatically reduces or increases thedrive signal applied to the detector when that signal tends to rise orfall in level, thus improving detector operation over a wide range ofsignal levels.

Another feature of the invention is the provision of a quadrature gridfrequency modulation detector wherein the potential at the screen gridof the detector tube increases or decreases the potential at the screengrid of a limiter tube driving the detector for maximizing sensitivityof the detector system to weak signals and reducing the sensitivelythereof to strong signals.

Further objects, features and the attending advantages (a! of theinvention will be apparent upon consideration of the followingdescription when taken in conjunction with the accompanying drawing inwhich:

FIG. 1 is a diagram of a television receiver incorporating the detectorsystem of the invention; and

FIG. 2 is a graph representing the intermediate frequency response ofthe receiver to the video and sound carriers of a television signal.

In practicing the invention there is provided a detector system forfrequency modulation signals including a limiter stage with an electronvalve having several control elements, or grids, which stage drives aquadrature grid detector. The detector includes an electron valve with acontrol element, or grid, driven by the signal to be detected and afurther control element, or grid, energized by a tuned circuit whichdevelops a control signal near phase quadrature to the applied signal sothat detection occurs as conduction of the tube is regulated by thephase difference at these two control elements. The detector valvefurther includes a control element or screen grid coupled throughresistor means to a potential source so that the voltage level at thisgrid varies inversely with the strength of the signal to be detected.This screen grid is also coupled to a grid, preferably the screen grid,of the limiter tube so that gain of this stage is regulated in inverserelation to the signal level applied to the detector stage whereby weaksignals are applied to the detector at an increased level and strongsignals are coupled thereto at a decreased level thereby minimizingdistortion in the detector and improving AM rejection.

FIG. 1 shows a television receiver including a radio frequency amplifierit which selects and amplifies signals which are then applied to themixer stage 12.. Local oscillater 14 is also coupled to mixer stage 12so that a received signal is thus heterodyned and applied to intermediate frequency amplifier 16 wherein it is selected and furtheramplified after which it is coupled to a detector stage 13. The videocarrier of the composite television signal is coupled to video amplifier2t and then to the cathode ray picture tube 22 which reproduces thissignal as an image on its screen. A synchronizing signal separator 25 isalso coupled to the video amplifier and this derives portions of thevideo signal which synchronize the horizontal sweep system 27 and thevertical sweep system 28 which are coupled to deflection yoke 30 toprovide scanning of the electron beam in the picture tube 22.

Detector 18 is also connected to a sound amplifier or driver 35 whichprovides amplification and limiting of the frequency modulation soundcarrier of the composits video signal, generally at 4.5 megacycles.Amplifier 35 is connected to the quadrature grid detector 37 whichdemodulates the frequency modulation carrier and applies the resultingaudio to the audio frequency amplifier 4t This audio signal is thencoupled to loudspeaker 42.

Amplifier it) and mixer stage 12 are tuned to the desired signal bymeans known in the art. Oscillator 14 is tuned to a frequency which isdisplaced from the desired signal by the intermediate frequency. Thecontrol for this purpose is represented diagrammatically herein asganged control 45. The oscillator 14 further includes a fine tuner 47which is operated by a control 48 in order to vary slightly thefrequency of the signal produced thereby for optimum reception. Theintermediate frequency amplifier 16 includes various tuned circuitsdesignated generally as tuned transformer 5t) which may have a frequencyresponse characteristic designated by curve 52 of FIG. 2. It should beapparent that as control 48 is varied the sound carrier 55 having a bandwidth 57 may be positioned at the peak or at some sloped portion of thecurve 52. Similarly, video carrier 59 having a band width s1 may also bepositioned on the peak or some sloped portion of curve 52. In thepresent day television signal video carrier 59 is separated by 4.5megacycles from sound carrier 55. In the situation illustrated in FIG.2, with video carrier 59 positioned on the peak of response curve 52, asmuch as 60 db can separate this carrier from sound carrier 55. Byrotuning fine tuner 47 it is possible that the reverse situation canexist with the level of the sound carrier exceeding the level of thevideo carrier by an amount of the order of 50 db.

Accordingly, over such wide signal input variations the level of thedriving si nal applied to detector 37 would ordinarily be undesirablylow in which case difficulty may be experienced in causing the detectorto lock in with the driving signal and this is particularly troublesomenear maximum deviation of the applied signal. Furthermore, at very highinput signal levels, overdrive of detector 37 can easily occur in whichcase the detector may have very poor AM rejection characteristics.

Referring to the frequency modulation detector system specifically, itmay be seen that the 4.5 megacycle sound carrier is applied fromdetector 18 to the control grid of driver electron valve 62. Cathodebias for this tube is provided by resistor 64 which is bypassed forsignal frequencies by capacitor 66. The suppressor grid of this tube isgrounded and the anode thereof is coupled through the primary Winding oftransformer 68 and decoupling resistor 7 to 3+. Capacitor 71 grounds B+for signals. The decoupling system is completed by a capacitor 72connected from resistor 70 to the screen grid of tube 62, vwhich screengrid is connected to the cathode through a bypass capacitor 73. Apositive potential for the screen grid of valve 62 is applied theretofrom B++ through resistor 75 and resistor 76 which are series connectedB++ is bypassed for signals by capacitor 78. Driver 35 is operated toprovide some limiting of the frequency modulation signal andamplification thereof to operate detector 57.

The secondary of transformer 6% is tuned by capacitor 77 and coupled tothe control grid of electron valve 79. The other side of the secondarywinding and capacitor 77 are connected to ground. Cathode bias for valve79 is provided by resistor 81 which is bypassed for signals by capacitor82. In a particular construction of the detector it may be advisable toconstruct this cathode circuit to be somewhat degenerative at the audiofrequencies involved to provide improved operation of the detector. Thescreen grid of valve 79 is connected to =B++ through resistor 75, whichis a resistor common to the screen grid energizing circuits of bothdriver and detector, and bypassed to ground through capacitor 84-. Afurther bypass for audio frequencies is provided by capacitor 86.Resistor 85, series connected with capacitor 86 reduces possibleresonant effects due to the presence of any inductive reactance incapacitor 86, which may be of the electrolytic type.

The suppressor grid of valve 79 is connected to one side of a paralleltuned circuit 87 which is shunted by a damping resistor 88. The otherside of tuned circuit 87 is connected to ground through a parallelcombination of bias resistor 90 and signal bypass capacitor 91. Tunedcircuit 87 is resonant at the frequency of the carrier of the soundintercarrier signal and is energized by this signal through couplingwithin the valve including the electrode capacity between the controlgrid and suppressor grid. In this circuit therefore, the suppressor gridwill be driven in phase quadrature to the frequency modulation signalapplied between the control grid and cathode. It is contemplated that atlow signal level the circuit will operate as a locked oscillatordetector due to the feedback through capacitance between the suppressorand control grids so that demodulated output appears at the anode ofelectron valve 79. At higher signal levels the input signal exceeds theself-oscillation level of the detector and the phase changes of thesignal on the supressor grid with respect to the driving signal at thecontrol grid, due to the tuned circuit 87, provide regulation ofconduction of the valve so that detected output appears at the anode. Aload resistor 95 is coupled from the anode of valve 79 to B++ and theanode is bypassed to ground for radio frequencies by capacitor 96. Thedemodulated output is then taken from the anode through a couplingnetwork comprising series connected resistor 97 and blocking capacitorW; which are connected to the audio frequency amplifier 40.

In the system of the present invention to overcome the previouslymentioned defects caused by wide variations in signal input level, thescreen grid of driver electron valve 62 is coupled to the screen grid ofdetector electron valve '79 and the two grids are coupled to anenergizing source through a common dropping resistor 75. Thus as thesignal level at the detector valve 79 increases, the screen grid currentincreases thereby increasing the voltage drop across resistor '75 andlowering the voltage at the screen grid of valve 62 which reduces theamplification in this stage and reduces the drive of the detector stage.Conversely as the signal at the detector decreases, the screen gridcurrent of valve 79 decreases producing a reduced voltage drop acrossresistor 75 thus raising the potential at the screen grid of the valve62 and increasing the amplification of this valve to increase the levelof the signal applied to the detector. By proper selection of componentvalues it is possible to greatly improve the detector operation.

In a constructed embodiment of the invention the part following circuitparameters were utilized and provided the operation as described:

Valve 62 3AU6 Resistor 64 ohms 150 Capacitor 66 microfarad .005 Resistor70 ohms 1000 Capacitor 72 microfarad .005 Capacitor 73 do .005 Resistor75 ohms 22,000 Resistor 76 do 10,000

Capacitor 77 and the secondary winding of transformer 68 are tuned tothe signal.

Valve 79 3DT6 Resistor 81 ohms 560 Capacitor 82 microfarad .01 Capacitor84 do .005 Resistor 85 ohms 220 Capacitor 86 microfarads 5 Tuned circuit87 is tuned to the frequency of the signal.

Resistor 88 ohms 82,000 Resistor 90 do 560,000 Capacitor 91 microfarad.01 Resistor 5 ohms 27,000 Capacitor 96 micromicrofarads 220 Resistor 97"ohms" 4700 Capacitor 98 microfarad .01 B+ volts B++ do 250 Accordinglyit may be seen that the detector system of the present invention willautomatically adjust itself for maximum drive under weak signalconditions in order to improve the lock-in characteristics of thedetector at such times and reduce the tendency for distortion at widedeviation of the applied signals. Furthermore, the system willautomatically adjust itself to prevent overdriving the detector stageunder strong signal conditions in order to provide improved detectoroperation under this condition. It should also be apparent that theseobjectives can be achieved in a simple manner and at low cost with thepresent invention.

I claim:

1. A system for detecting frequency modulation signals including incombination a limiter stage having a first electron valve with aplurality of electrodes including a space current drawing electrode thepotential of which regulates the gain of said limiter stage, means toapply frequency modulation signals to said limiter stage, a detectorstage including a second electron valve having a plurality of gridsincluding a screen grid, said detector stage also including circuitmeans forming with said second electron valve a quadrature frequencymodulation demodulator, circuit means connecting said limiter stage tosaid detector stage for applying signals to said detector stage, directcurrent conducting means including a resistor coupling said screen gridof said detector stage to a potential source so that the potential atsaid detector screen grid varies with the level of said signals, andfurther direct current conducting means coupling said current drawingelectrode of said limiter stage to said screen grid of said detectorstage so that the gain of said limiter stage is regulated ac cording tothe level of signals being detected.

2. In a television receiver which translates a frequency modulated soundsignal of substantially fixed frequency and variable strength, thesystem for detecting said signal including in combination a driver stagehaving a first electron valve with a plurality of grids including ascreen grid the potential of which regulates the gain of said driverstage, means for applying said sound signal to said driver stage, adetector stage including a second electron valve having a plurality ofelectrodes including an output electrode, said detector stage alsoincluding circuit means providing quadrature grid frequency modulationdetection in said second electron valve, circuit means coupling saiddriver stage to said detector stage for applying said sound signalthereto, resistor means coupling said output electrode of said secondelectron valve to a positive potential source, the value of saidresistor means being selected so that changes in signal level at saiddetector stage are reflected as a change in potential at a point of saidresistor means, and direct current conducting means coupling said screengrid of said first electron valve to said point of said resistor meansso that the gain of said river stage is regulated according to thestrength of signals being detected.

3. In a television receiver which translates a frequency modulated soundsignal of substantially fixed frequency and variable strength, thesystem for detecting said signal including in combination a limiterstage having a first electron valve with a plurality of electrodesincluding a space current drawing electrode the potential of whichregulates the amplification of said limiter stage, means for applyingsaid sound signal to said limiter stage, a detector stage including asecond electron valve having a plurality of grids including a screengrid, said detector stage having circuit means including a tuned circuitcoupled to a grid of said second electron valve providing quadraturegrid frequency modulation detection in said electron valve, circuitmeans coupling said limiter stage to said detector stage for applyingsaid sound signal thereto, first resistor means coupling said screengrid of said second electron valve to a positive potential source, thevalue of said first resistor means being selected so that changes insignal level at said detector stage are reflected as a change inpotential at a point of said first resistor means, and second resistormeans coupling said space current drawing electrode of said firstelectron valve to said point of said first resistor means so that thegain of said limiter stage is regulated according to the strength ofsignals being detected.

4. A system for detecting frequency modulation signals including incombination a driver stage including a first electron valve having firstand second control elements and a first output element, means to applyfrequency modulation signals to said first control element so that saidsignals appear at said first output element, said second control elementbeing adapted to regulate the gain of said driver stage according to thepotential thereof, a detector stage including a second electron valvehaving third, fourth and fifth control electrodes and a second outputelectrode, circuit means coupling said third control element to saidfirst output element, a tuned circuit coupled to said fourth controlelement and energized by said signals so that phase changes of saidsignal in said tuned circuit cause detected signals to appear at saidsecond output element, resistor means for coupling said fifth controlelectrode to a potential source so that the potential of said fifthcontrol element is dependent upon the level of said signals, and directcurrent conducting means including signal decoupling means coupling saidsecond control electrode of said first electron valve to said fourthcontrol electrode of said second electron valve so that the gain of saiddriver stage is regulated according to the level of signals to bedetected.

5. In a system for demodulating frequency modulated carrier waves,apparatus comprising the combination of a limiter amplifier forfrequency modulated carrier waves, said limiter amplifier including anelectron discharge device having a cathode, a control grid, a screengrid and an anode, an input circuit coupled between said control gridand cathode to which said frequency modulated carrier waves are applied,an output circuit coupled between said anode and cathode, frequencymodulation detecting means coupled to said output circuit, saiddetecting means including a second electron discharge device operatingas a quadrature grid detector for detecting directly modulated carrierwaves of relatively large amplitude and for detecting lockedoscillations when the modulated carrier waves are of relatively smallamplitude, means for deriving from said detecting means a controlvoltage which varies inversely with the amplitude of said frequencymodulated carrier waves, and means for applying said control voltage tosaid screen grid.

6. Apparatus in accordance with claim 5 wherein said second electrondischarge device includes a screen electrode which draws a largercurrent when said modulated carrier waves are of relatively largeamplitude than when said modulated carrier waves are of relatively smallamplitude, and wherein said control voltage deriving means is responsiveto the current drawn by said screen electrode.

7. In a television receiver, an intercarrier' sound channel comprisingthe combination of a source of intercarrier sound intermediate frequencysignals, a sound intermediate frequency limiter amplifier coupled tosaid source, a frequency modulation detector coupled to said soundintermediate frequency limiter amplifier, said frequency modulationdetector comprising an electron discharge device operating as anoscillating detector when the output signal level of said soundintermediate frequency limiter amplifier is relatively low and operatingas a non-oscillating detector when the output signal level of said soundintermediate frequency limiter amplifier is relatively high, means forderiving a control voltage from said detector which increases when achange in said output signal level causes said detector to change fromoperation as a non-oscillating detector to operation as an oscillatingdetector and which decreases when a change in said output signal levelcauses said electron discharge device to change from operation as anoscillating detector to operation as a non-oscillating detector, andmeans for automatically controlling the gain of said sound intermediatefrequency limiter amplifier in accordance with said control voltage.

8. In a television receiver provided with a source of sound intermediatefrequency signals, the combination comprising a sound intermediatefrequency limiter amplifier coupled to said source, frequency modulationdetecting means coupled to said sound intermediate frequency limiteramplifier, said sound intermediate frequency limiter amplifiercomprising an electron discharge device including a screen gridelectrode, said detecting means comprising a second electron dischargedevice opcrating as a quadrature grid detector for detecting directlymodulated carrier waves of relatively large amplitude and for detectinglocked oscillations when the modulated carrier waves are of relativelysmall amplitude, said second electron discharge device also including ascreen grid electrode, means including a resistor for connecting saidsound detector screen grid electrode to a source of op eratingpotential, and means including said resistor for connecting said soundintermediate frequency limiter amplifier screen grid electrode to saidsource of operating potential,

9. In a television receiver, the combination comprising an intermediatefrequency signal channel common to received sound and picture signals, avideo detector coupled to said intermediate frequency channel, means forderiving an intercarrier sound signal from said video detector output, alimiter stage comprising a first electron discharge device coupled tosaid deriving means, a frequency modulation detector coupled to saidlimiter stage and responsive to the output thereof, said frequencymodulation detector comprising a second electron discharge deviceoperating as a quadrature grid detector for detecting directlyintercarrier sound signals when the intercarrier sound signal output ofsaid limiter stage is of relatively large amplitude and for detectingoscillations locked to said intercarrier sound signals when theintercarrier sound signal output of said limiter stage is of relativelysmall amplitude, said first and second electron discharge devices eachincluding a screen grid, means including a resistor for connecting thescreen grid of said second electron discharge device to a source ofrelatively fixed operating potential, the current drawn by the screengrid of said second electron discharge device through said resistorbeing appreciably larger when said second electron discharge deviceresponds to intercarrier sound signals of relatively large amplitudethan when said second electron discharge device responds to intercarriersound si nals of relatively small amplitude, and means including saidresistor for connecting the screen grid of said first electron dischargedevice to said source of operating potential.

10. In a system for demodulating frequency modulated carrier waves,apparatus comprising the combination of an amplifier for said frequencymodulated carrier waves, said amplifier including an electron dischargedevice having a cathode, a control grid and an anode, an input circuitcoupled between said control grid and cathode to which said frequencymodulated carrier waves are applied, an output circuit coupled betweensaid anode and cathode, frequency modulation detecting means coupled tosaid output circuit and operating as an oscillating detector withrespect to frequency modulated carrier Waves appearing in said outputcircuit at amplitude levels in a first predetermined range, saiddetecting means operating as a nonoscillating detector with respect tofrequency modulated carrier waves appearing in said output circuit atamplitude levels in a second predetermined range higher than said firstpredetermined range, means for deriving from said detecting means acontrol voltage which is of a greater magnitude when said detectingmeans operates as an oscillating detector than when said detecting meansoperates as a non-oscillating detector, and means coupled to saidamplifier for controlling the gain of said amplifier in accordance withthe control voltage derived by said deriving means.

References Cited in the file of this patent UNITED STATES PATENTS1,949,507 Wise Mar. 6, 1934 1,971,741 Willging Aug. 28, 1934 2,343,263Okrent Mar. 7, 1944 2,368,052 Unger Jan. 23, 1945 2,494,795 Bradley Jan.17, 1950 2,874,277 Orr Feb. 17, 1959 FOREIGN PATENTS 639,922 GreatBritain July 12, 1950

5. IN A SYSTEM FOR DEMODULATING FREQUENCY MODULATED CARRIER WAVES,APPARATUS COMPRISING THE COMBINATION OF A LIMITER AMPLIFIER FORFREQUENCY MODULATED CARRIER WAVES, SAID LIMITER AMPLIFIER INCLUDING ANELECTRON DISCHARGE DEVICE HAVING A CATHODE, A CONTROL GRID, A SCREENGRID AND AN ANODE, AN INPUT CIRCUIT COUPLED BETWEEN SAID CONTROL GRIDAND CATHODE TO WHICH SAID FREQUENCY MODULATED CARRIER WAVES ARE APPLIED,AN OUTPUT CIRCUIT COUPLED BETWEEN SAID ANODE AND CATHODE, FREQUENCYMODULATION DETECTING MEANS COUPLED TO SAID OUTPUT CIRCUIT, SAIDDETECTING MEANS INCLUDING A SECOND ELECTRON DISCHARGE DEVICE OPERATINGAS A QUADRATURE GRID DETECTOR FOR DETECTING DIRECTLY MODULATED CARRIERWAVES OF RELATIVELY LARGE AMPLITUDE AND FOR DETECTING LOCKEDOSCILLATIONS WHEN THE MODULATED CARRIER WAVES ARE OF RELATIVELY SMALLAMPLITUDE, MEANS FOR DERIVING FROM